1. Field of the Invention
The present invention relates to a power source circuit, a power source for driving a liquid crystal display, and a liquid crystal display device, and more particularly, to a new structure for a multiple output power source circuit which can supply a plurality of suitable electric potentials as a power source for driving the liquid crystal panel in a liquid crystal display device.
2. Description of Related Art
Conventionally, power source circuits which supply a plurality of electric potentials have been used for the driving circuit in liquid crystal panels, and one example of these power source circuits is disclosed in Japanese Laid-Open Patent Publication Hei 2-150819. FIG. 11 shows the basic structure of this conventional power source circuit. In the liquid crystal panel 1, a plurality of parallel segment electrodes SE1, SE2, . . . , (hereafter abbreviated as SEn) which extend in stripe form, and a plurality of parallel common electrodes CE1, CE2, . . . (hereafter abbreviated as CEn) which extend in a direction orthogonal to the segment electrodes, are provided in a state facing each other with an unrepresented liquid crystal layer interposed in between. The areas of the liquid crystal layer where these segment electrodes SEn and the common electrodes CEn cross comprise pixels, the optical state of which can change and which can be controlled to be dark or bright, and through the plurality of pixels, a desired display state can be reproduced over the liquid crystal panel as a whole.
When the attempt is made to display a desired picture image on the liquid crystal panel 1, specific electric potentials are impressed for a specific length of time by a liquid crystal driving circuit in order to form the pixel state corresponding to the picture image display on the segment electrodes SEn and the common electrodes CEn, and through so-called time division driving, the state of each pixel is controlled, said pixels having a structure which is equivalent to a condenser with the liquid crystal layer interposed between electrodes.
The circuit shown in FIG. 11 is a multiple output power source circuit which is used to supply the electric potentials V0, V1, V2, V3, V4 and V5 to the driving circuit of the liquid crystal panel 1. In this circuit, first, using the high electric potential VDD, which is the power source electric potential that is supplied from the power source, and the low electric potential VEE as a base, the voltage is divided by voltage dividing resistors R1, R2, R3, R4 and R5, to form intermediate electric potentials V1, V2, V3 and V4. These intermediate electric potentials V1, V2, V3 and V4 are input into the noninverting input terminals of the operational amplifiers OP1, OP2, OP3 and OP4 which are formed inside the integrated circuit 2. These operational amplifiers OP1, OP2, OP3 and OP4 are composed as voltage followers with the output terminals and inverting input terminals short circuited, and can supply the intermediate electric potentials V1, V2, V3 and V4 with low output impedance.
The output side of the operational amplifiers OP1, OP2, OP3 and OP4 are connected to resistors R8, R9, R10 and R11, respectively, and the resistors R8 through R11 restrict the output current of the operational amplifiers OP1 through OP4. In addition, after this, the top three electric potentials, out of the six electric potentials including the power source electric potential VDD and VEE, and the bottom three electric potentials are connected by condensers C1, C2, C3 and C4 between the respective electric potentials.
From the power source circuit thus formed, six output electric potentials V0 to V5 are output, with the power source electric potentials VDD as V0 and VEE as V5. These output electric potentials V0 through V5 are impressed on the respective segment electrodes SEn and common electrodes CEn through the liquid crystal driving circuit which acts in accordance with the field signal corresponding to the picture image.
The voltage levels necessary when the liquid crystal panel is time division driven with high duty by the voltage averaging law are generally as shown in FIG. 12, and are the output electric potentials V0 to V5 having the relationships EQU V0-V1=V1-V2=V2-V3=V3-V4=V4-V5 (1)
(here, V0&gt;V1&gt;V2&gt;V3&gt;V4&gt;V5).
The signals which are applied to the segment electrodes SEn and the common electrodes CEn are, for example, as shown in FIG. 12. In FIG. 12, the signal electric potential which is impressed on the segment electrodes SEn and is indicated by the dashed lines switches to either V3 or V5 within the interval of frame 0 (hereafter called Fr0) shown in FIG. 12, and in addition, switches to either V0 or V2 in the interval of frame 1 (hereafter called Fr1) shown in FIG. 12. For example, the signal electric potential V0 corresponds to the on state of the corresponding pixel region, and the signal electric potential V2 corresponds to the off state. The switching state between the electric potential levels of the segment electrodes SEn changes depending on the pattern displayed.
On the other hand, the signal electric potential impressed on the common electrodes CEn is normally the non-selective state of V4 in the interval of Fr0, and becomes the selective state of V0 for only a specific interval. In addition, in the interval of Fr1, the electric potential is normally the non-selective state of V1, and becomes the selective state of V5 for only a specific interval. The interval over which the common electrodes CEn achieve the selective state differs for each common electrode, and in general, the plurality of common electrodes CEn do not achieve the selective state simultaneously.
The intervals of Fr0 and Fr1 shown in FIG. 12 alternatingly repeat, and through this the liquid crystal layer in the pixel areas undergoes alternating current driving, thereby preventing deterioration of the liquid crystal layer.
When the electric potential levels of these kinds of segment electrodes SEn and common electrodes CEn switches, the capacitance (composed of the segment electrode, the common electrode and the liquid crystal layer interposed therebetween) of the pixels which exist in plurality in the liquid crystal panel is charged and discharged, and consequently, an electric current is created between each of the electric potential levels of the output electric potentials V0 through V5 of the power source circuit through the liquid crystal panel. At this time, the switching of the electric potential level of the segment electrodes SEn is accomplished between V0 and V2, or between V3 and V5, and in addition, the majority of the common electrodes CEn are in a non-selective state, being the electric potential level of either V1 or V4. Accordingly, the electric current accompanying the switching of the electric potential levels of the segment electrodes SEn primarily flows between V0, V1 and V2, and between V3, V4 and V5. In contrast to this, the common electrodes CEn are, as described above, for the most part in a non-selective state being the electric potential level of either V1 or V4, but this becomes the electric potential level of V0 or V5 when the selective state is achieved. Accordingly, the electric current accompanying switching of the electric potential levels of the common electrodes primarily flows between V0, V3, V4 and V5, and between V0, V1, V2 and V5.
The current which is generated in the power source circuit created when the liquid crystal panel 1 is driven using this type of electric current, that is to say the above-described power source circuit, is supplied as a portion of the electric current which flows from the power source electric potential VDD to VEE. In other words, when considering, for example, the electric current which flows from the electric potential level V3 to V4 in the liquid crystal panel accompanying the switching of the electric potential levels of the segment currents SEn, this electric current flows initially out from the power source electric potential VDD, as shown in FIG. 11, and flows across the operational amplifier OP3 into the liquid crystal panel 1 at the electric potential level V3, returns to the electric potential level V4 from the liquid crystal panel 1 and flows finally to the power source electric potential VEE via the operational amplifier OP4. Accordingly, when the power source circuit shown in FIG. 11 supplies an electric current which flows out from the output electric potential V3 to the liquid crystal panel 1 and returns to V4, the power consumption caused by the electric current that flows from the power source electric potential VDD to the output electric potential V3, and the power consumption caused by the electric current that flows from the output electric potential V4 to the power source electric potential VEE is only that of generating heat in the operational amplifiers OP3 and OP4, and there is no effective work with respect to the liquid crystal panel 1, so that there is no wasted power consumption.
The electric current which is generated accompanying the switching of the electric potential levels of the segment electrodes SEn flows primarily between V0, V1 and V2, and between V3, V4 and V5, while the electric current which is generated accompanying the switching of electric potential levels of the common electrodes CEn flows primarily between V0, V3, V4 and V5, and between V0, V1, V2, and V5, and consequently, the former has a smaller voltage between each electric potential level than the latter. Accordingly, in contrasting the supplying of electric current accompanying the switching of the electric potential levels of the segment electrodes SEn and the supplying of electric current accompanying the switching of the electric potential levels of the common electrodes CEn using the power source circuit of FIG. 11, the division of power which is consumed in the liquid crystal panel 1 is smaller in the former than in the latter with respect to the above-described wasted power consumption, and consequently, more power is wasted.
In recent years, demand for larger capacity and faster liquid crystal display panels has risen, and the shift to high duty in time-division driving of liquid crystal panels for this purpose has been dramatic. In order to increase the duty ratio during driving in this way, a larger voltage is necessary as the power source voltage and the electric potential difference between the high electric potential VDD and the low electric potential VEE expands, and consequently, the following problems are created in the conventional power source circuit shown in FIG. 11.
(1) Because the above-described power source electric potentials VDD and VEE are used as the power source of the operational amplifiers, the power consumption which is caused by the operational amplifier idling current which flows steadily increases because of the expansion of this electric potential difference.
(2) Because of the rise in power source voltage, it is necessary to use expensive, high voltage-resistance operational amplifiers as the operational amplifiers used in the power source circuit.
(3) Because of the rise in power source voltage, the wasted amount of power which is consumed in the above-described power source circuit, in particular the wasted power consumption which is created when the electric current accompanying switching of the electric potential levels of the segment electrodes SEn is supplied, increases.
Thus, in consideration of the foregoing problems, it is an objective of the present invention to compose a power source circuit which has low power consumption and moreover is an inexpensive power source circuit, and in particular is suitable as the power source for driving a liquid crystal display, and through utilizing such a power source circuit, to reduce power consumption in the liquid crystal display device as a whole and to reduce production costs.